Means for increasing electron density in low pressure gas discharge tube



March 12, 1968 c. o. LUSTIG 3,373,304

MEANS FOR INCREASING ELECTRON DENSITY IN LOW PRESSURE GAS DISCHARGE TUBE Filed Aug. 20, 1965 2 Sheets-Sheet l I W m 1 1 4 I I I 1 //1 I I 1 I f I I :f I I I I I I I I I L l 4 E g C 3 17 E 16 bL *4 STARTER NOISE INTENSITY (MAX. LEVEL 10 'WATTS/M SEC.

L l u MAGNETIC FIELD (GAUSS) .FIG.5.

INVENTOR. CLAUDE D. Lusr/a A TTO/P/VEY ELECTRON DENSITY (lo /cc) March 12, 1968 c. D. LUSTIG 3,373,304

MEANS FOR INCREASING ELECTRON DENSITY IN LOW PRESSURE GAS DISCHARGE TUBE Filed Aug. 20, 1965 2 Sheets-Sheet 2 -N0 APERTURE APERTURE= 9 MM MA 12 A-APERTURE: 6 MM 01A.

(CONSTANT CURRENT) m MAGNETIC Fl LD=450GAUSS 5 Q 5 APERTURE DIA.= 6 MM 2 TUBE DIA.= 17 MM 4. 8 4-- NO MAGNETIC FIELD 1 1-- NOAPERTURE b I I I I 0 I I 0 100 200 300 400 500 0 .05 .10 15 .20

MAGNETIC FIELD (GAUSS) C RRENT (AMPERES) F I G. 2. F l G. 3.

APERTURE 6 MM DIA. 1 MICRON PRESSURE 9- '8 2 6-- t 2 LIJ a 4" 2 O n: 3-- Q j 2 APERTURE-G MM DIA. w 10 MICRONS PRESSURE o I i I I i MAGNETIC FIELD (GAUSS) INVENTOR.

FIG.4.

CLAUDE D. Lusr/a BY QJL 14- $1M A fro/ EI United States Patent 3,373,394 MEANS FGR INCREASING ELECTRQN DENITY EN LOW PRESSURE GAS DISCHARGE TUBE lauds D. Lustig, Arlington, Mass, assignor to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed Aug. 20, 1965, Ser. No. 481,340 6 Claims. (Cl. 313161) ABSTRACT 0F THE DECLOSURE An elongated electron discharge tube having a constriction in the electron path and a solenoidal magnet for. producing a longitudinal unidirectional magnetic field in a region between the constriction and the anode of the discharge tube.

This invention relates to gas filled electron discharge tubes operating at relatively low gas pressures, and in particular the invention relates to means for increasing the electron density in the ionized medium of such a device, thereby to enhance the radiation of energy from the device.

Gas filled electron discharge tubes may find use as coherent or incoherent light sources, as electrical signal sources, and as electrical noise sources. In most of these devices, an increase in the electron density results in an increase in the particular type of useful radiation concerned. One means for increasing the electron density is to increase the electron current in the tube by the use of a larger power supply. As the current is increased the size and cost of the external power supply increases rapidly and the thermal operating temperature of the tube increases, and this often necessitates additional cooling equipment. Therefore, it is obvious that means for increasing the electron density in a gas filled electron discharge tube without the above-mentioned undesirable consequences will be of considerable benefit.

In gas discharge tubes whose gas pressures are in the range of approximately microns and above, it has been reported that the electron density within a tube has been increased by immersing the tube within an axially-directed unidirectional magnetic field. This alleged increase in electron density apparently does not result from any significant increase in. the total number of free electrons but occurs only along the axis of the magnetic field and results from substantially the same number of electrons being concentrated along the magnetic field fiux lines.

According to the present invention I am able to significantly increase the total number of free electrons and thus the electron density in alow pressure gas filled electron discharge tube that is immersed in a unidirectional magnetic field by restricting the cross-sectional area of the discharge tube in a narrow region adjacent one end of the tube. The electron density was determined from the frequency shift of a microwave resonant cavity. This method gives a value'for the total number of electrons within a given length or" the discharge, independently of the radial density distribution.

It isan object of this invention to provide simple means for increasing the number of free electrons in the plasma of a low pressure gas filled discharge tube without anaccompanying increase in the electrical power source required for establishing the discharge.

Another object of this invention is to increase the electron density within a relatively low pressure gas discharge tube which is immersed in a unidirectional magnetic field by adding a simple physical structure within the tube.

The invention will be described by referring to the accompanying drawings wherein:

FIG. 1 is a simplified illustration of an arrangement of a low pressure gas discharge tube having a restricting aperture therein and operated in accordance with the present invention to produce enhanced radiation of microwave energy;

FIG. 2 is a series of curves that illustrate the efiect that aperture size has on the electron density within the tube;

FIG. 3 is a graph that illustrates the electron densities that may be achieved with a gas discharge tube operated in accordance with the present invention as compared to the electron densities that are achieved without the use of an applied magnetic field and without the inclusion of an apertured dielectric disc within the discharge tube;

FIG. 4 is a graph illustrating the effect that gas pressure within the tube has upon the electron density within the tube; and

FIG. 5 is a curve that illustrates the intensity of the noise radiated from a gas discharge tube constructed and operated in the manner illustrated in FIG. 1.

Referring now in detail to FIG. 1, the apparatus includes a gas discharge tube 11 which may be made of a dielectric material such as quartz or glass and which is filled with an ionizable gas such as argon at a low pressure of approximately one micron. Tube 11 is provided with an electron emissive cathode 12 at one end and an electron collector electrode 13 at the opposite end. Cathode 12 may be an oxide-coated nickel mesh cathode which is maintained at an operating temperature of approximately 960 C. by means of the cathode battery E A potential is maintained between cathode 12 and collector 13 by means of a direct current biasing source E, which in one operating embodiment of the invention provided a potential that ranged between and 200 volts. In that embodiment of the invention the tube was approximately one meter long, although the tube length is not critical. A discharge may be initiated within the discharge tube 11 by means of the starter i6, and the resistor 17 serves to stabilize the discharge. The gas within the discharge tube 11 is ionized by the electron discharge established between cathode 12 and collector 13. A microwave resonant cavity 29 is disposed about the central region of discharge tube 11 and the coupling loop 21 serves to couple generated electromagnetic waves from cavity 20, the waves being generated by the ionized medium within the discharge tube. A solenoid 25 is disposed about cavity 20 and supplies a unidirectional magnetic field that is directed along the central axis of discharge tube 11. Discharge tubes filled with argon, helium, and neon gases have been used successfully, but other ionizable gases also may be employed as the ionizable medium.

In a localized region between cathode 12 and the left end of solenoid 25 there is disposed within the tube a thin disc 27 or" a dielectric material which has a central aperture 28 therein. In one embodiment of the invention the elongated body portion of the gas discharge tube 11 had an inner diameter of 17 millimeters and the aperture 28 of the disc 27 had a diameter of 6 millimeters. The disc 27 was made of quartz and had a thickness of 3 millimeters.

I have found that the apertured disc 27 within the gas discharge tube 11 causes a significant increase in the elec tron density throughout the ionized medium, and this is achieved without an increase in the gas pressure and without an increase in the potential sources for providing the current dischargethrough the tube. The electron density within gas discharge tube 11 as a function of the magnetic field strength provided by the solenoid 25, and with the diameter of the aperture 28 as a parameter is illustrated in the graphs of FIG. 2. Curve a of FIG. 2 is a plot of the electron density asa function of applied magnetic field strength with no apertured disc at all within the discharge tube 11. Curve b of FIG. 2 illustrates the increased electron density that was obtained with a dielectric disc 27 that had an aperture 28 of 9 millimeters in diameter. Curve illustrates the electron density that was obtained when the disc 27 had an aperture 28 of 6 millimeters in diameter. It will be seen in FIG. 2 that the smaller diameter aperture produced a significantly increased electron density which in the range of 300 gauss magnetic field strength, for example, was approximately three times as great as the electron density that existed with no apertured disc within the gas discharge tube. Even greater electron densities may be obtained by using restricting apertures with diameters smaller than 6 millimeters.

The manner in which the electron density within the gas discharge tube having an apertured disc therein changes as a function of current through the tube is illustrated in FIG. 3. Curve a of FIG. 3 illustrates the rapid increase in electron density as a function of current for the condition in which the applied magnetic field had a strength of approximately 450 gauss and the diameter of aperture 28 was 6 millimeters. For comparison purposes, curve b of FIG. 3 illustrates the change in electron density as a function of current for the same gas discharge tube which had no apertured disc 27 therein and to which no unidirectional magnetic field was applied. Curve a shows that an electron density of approximately 10 electrons per cubic centimeter was achieved with a current of approximately .17 ampere. An extrapolation of the curve b of FIG. 3 indicates that a current of approximately 4.0 amperes would be required to obtain an electron density of 10 electrons per cubic centimeter.

The exact mechanism by which the high electron densities are created Within the ionized medium (plasma) of the discharge tube is not fully understood. However, in an attempt to analyze the situation, it will be remembered that the following two conditions must be satisfied in any active steady state discharge: (1) I='nev where I is the current density, n is the electron density, e is the electronic charge, and v is the electron drift velocity; (2) the rate of loss of electrons by recombination with positive gas ions, which occurs mostly at the wall of the discharge tube, must equal the rate of production of ions. The data obtained for the curves of FIG. 2 was for the condition of constant current, so it may be reasoned that since the electron density n increased with increasing magnetic field strength, the electron drift velocity v must have decreased. This suggests, and it has been confirmed experimentally, that there is a low electric field in the region of the plasma where the electron density is high. It also was observed that the total voltage drop measured along the length of the tube was not significantly reduced as the magnetic field strength was increased. This suggests that a large voltage drop existed at the discontinuity that was produced bythe apertured disc 27. This voltage drop accelerates to a high energy some of the plasma electrons, which; have relatively long mean free paths due to the low gas pressure, and it is believed that these higher energy electrons more readily ionize the neutral gas atoms and thus are responsible for'the observed enhancement of the electron density. The resulting high density plasma contains these primary electrons so that a nonequilibrium,

distribution of electron energies exists in the high density region. The electron density attains a value such that the rate of loss of electrons to the wall, which. is inherently low because of. the magnetic field, equals the rate of ionization (principally by the high energy electrons), as required by condition (2) above.

The effect that 'gas pressure has upon the electron density within the plasma at discharge tube 11 having the apertured disc 27 therein is illustrated by the curves of FIG. 4. The sharply rising curve a of FIG. 4 is a replot' of the curve 0 of FIG. 2 in which an apertured disc having an aperture diameter of 6 millimeters was included Within the discharge tube that was filledwith argon gas at a pressure of one micron. Curve b of FIG. 4 illustrates the electron density that was obtained in the same tube having the same sized apertured disc therein but where the pressure of the argon gas was approximately 10 microns. It is evident from FIG. 4 that the electron density of the lower pressure tube increased quite sharply to a high value as a function of magnetic field strength while the electron density of the higher pressure tube increased rather slightly. With an argon filled tube I have determined that a significant increase in electron density cannot be achieved with a gas pressure much in excess of approximately 5 microns. It is believed that no appreciable increase in the total number of electrons within the plasma is achieved within a higher pressure discharge tube because the higher population of neutral atoms within the tube causes the mean free paths of the electrons to be relatively short so that they cannot achieve any appreciable increase in energy from the voltage across aperture 28, with the result that no significant increase in ionization of the neutral gas atoms is possible.

Because the increased electron density that is obtained from the device of FIG. 1 gives rise to enhanced radiation of energy, the present invention will be useful to increase the energy output of such gas discharge devices as microwave noise sources, microwave signal sources, laser devices, and incoherent light sources, for example. An example of the enhanced noise radiation from a gas discharge tube of the type illustrated in FIG. 1, with a 6 millimeter diameter aperture, is illustrated in FIG. 5. The peaks of noise radiation occur near multiple harmonics of the electron cyclotron frequency with the discharge tube filled with argon gas at a pressure of approximately one micron. The maximum noise intensity, which occurs in the vicinity of the fourth harmonic, was at a level of approximately 10 watts per megacycle per second. For the same discharge tube without an apertured disc 27, the electron density was lower and no radiation peaks were observed.

While I have described my invention in its preferred embodiments, it is to be understood that the words which I have used are words of description rather than of limit-ation and that changes within the purview of the appended claims may be made Without departing from the true scope and spirit of my invention in its broader aspects.

What is claimed is:

1. Means'for producing high electron densities in. an electron discharge device having an ionizable medium therein comprising the combination,

an elongated container for confining the volume of an ionizable gas, 7 means including an anode and a cathode for establishing an electron discharge through said gas in a direction parallel to the longitudinal dimension of said container,

an apertured dielectric disc disposed within saidcontainer transversely to said discharge, and means for establishing a unidirectional magnetic field directed along the axis of said discharge tube in the region between said disc and said anode, said means being further arranged to provide only fringing flux lines in the region of said cathode.

2. The combination claimed in claim 1 wherein said apertured disc is disposed adjacent the end of said container from which said electron discharge emanates.

3. Means for increasing electron density in a low pressure gas discharge tube comprising the combination,

an elongated closed tube of dielectric material,

an ionizable gas confined within said tube,

means including an anode and a cathode for establishing an electron discharge along the axis of said tube,

a thin centrally apertured dielectric disc concentrically 5 disposed Within said tube, said disc being further disposed adjacent the cathode, and

solenoid means concentric with said elongated closed tube for establishing a unidirectional magnetic field along said axis, said solenoid means further being disposed longitudinally so as to be completely within the region intermediate the disc and the anode.

4. The combination claimed in claim 3 wherein said unidirectional magnetic field has a strength in excess of approximately 50 gauss.

5. The combination claimed in claim 4 wherein said ionizable gas is confined within said tube at a pressure below approximately 5 microns.

6. An electronic tube for operating with a gas plasma having an increased number of free electrons therein, the combination comprising,

an elongated gas container having a given cross-sectional area,

an ionizable gas within said container at a pressure not in excess of approximately five microns,

means including an anode and a cathode for establishing an electron discharge through said medium to ionize neutral particles of said gas,

solenoid means disposed concentrically about said elongated container for immersing said gas in a longitudinally directed unidirectional magnetic field having a field strength in excess of approximately fifty gauss, and

an apertured dielectric disc positioned Within said container for restricting the cross-sectional area of said container within a localized region, said solenoid means extending longitudinally between first and second transverse planes intermediate said disc and said anode.

References Cited UNITED STATES PATENTS 1,954,025 4/1934 Reynolds 313204 X 2,004,175 6/1935 Schottky 3l3204 2,217,186 10/1940 Smith 313161 JAMES W. LAWRENCE, Primary Examiner.

P. C. DEMEO, Assistant Examiner. 

